Interactions between Scots pine, Ips acuminatus (Gyll.) and Ophiostoma brunneo-ciliatum (Math.): estimation of the critical thresholds of attack and inoculation densities and effects on hydraulic properties in the stem

The aggressiveness towards Scots pine (Pinus sylvestris L.) of the association between a bark beetle (Ips acuminatus Gyll) and an Ophiostomatale fungus (Ophiostoma brunneo-ciliatum Math.) was investigated by estimating experimentally with young trees, the critical threshold of attack or inoculation densities.Records of the relationship between natural attack densities by the beetles and survival of trees in a pine stand yielded a critical attack density threshold of about 900 m -2 .Experimental mass inoculations of young pines with the fungus, in a forest stand in Central France, demonstrated a weak pathogenicity of this fungal species towards Scots pine.Inoculation densities varying from 200 to 1000 m -2 were used.Damage in the bark or in the sapwood recorded three months after the inoculations, remained rather limited.The length of the induced reaction zones in the bark was small as compared to those obtained with more aggressive fungi, and did not increase with inoculation density.Damage in the sapwood, estimated either visually from the observed sapwood drying, and from resinosis, remained limited, but increased significantly with the inoculation density.The impairment of hydraulic conductivity of inoculated trunk segments was rather large with, at highest densities, a loss of conductivity estimated to about 60%.Nevertheless, due to the fact that the resistance to water transfer in the trunk is much smaller than in other organs (like roots or needles), this increase probably only had a small impact on water relations at whole tree level.It may be concluded that the association I. acuminatus -O.brunneo-ciliatum displays only a weak aggressiveness towards Scots pine, and that high densities of attacks or inoculations (above 1000 m -2 ) are required in order to reach the critical threshold able to kill trees. Pinus sylvestris / Ips acuminatus / Ophiostoma brunneo-ciliatum / inoculation density / attack density / resistance level / induced reaction zones / phloem / sapwood / water conductivity / bark-beetleRésumé -Relations Pinus sylvestris L. / Ophiostoma brunneo-ciliatum Math./ Ips acuminatus Gyll.: seuils critiques de densités d'attaques et d'inoculations ; impact d'inoculations massives sur les propriétés hydrauliques des tiges.La sensibilité de pins sylvestres (Pinus sylvestris L.) aux attaques de scolytes (Ips acuminatus) et de leur champignon associé (Ophiostoma brunneociliatum) a été étudiée en estimant les seuils critiques de densité d'attaques et d'inoculations sur de jeunes arbres.La relation entre la densité d'attaques naturelles et la survie des arbres a permis de déterminer un seuil critique de densité d'attaques d'environ 900 m -2 .